Steam generator nondestructive examination method

ABSTRACT

A method of examining a steam generator heat exchange tube from the outside surface employing ultrasonic nondestructive inspection techniques. An ultrasonic transducer contacts the outside surface of the tube and transmits a pseudo helical Lamb wave into the wall of the tube chosen to have a mode that does not significantly interact with water in the tube. The reflected waves are then analyzed for changes in modes to identify defects in the wall of the tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.11/562,782, filed Nov. 22, 2006 and is related to and claims priorityfrom provisional application Ser. No. 60/740,061, filed Nov. 28, 2005entitled Wear Scar Characterization For The Nuclear Industry.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to the nondestructive examination ofmetal tubes and more particularly to the nondestructive examination ofheat exchanger tubes from the secondary side of a steam generator tocharacterize wear scars on the heat exchanger tubes that may warrantfurther examination from the primary side.

2. Description of the Prior Art

Steam generators used in nuclear reactor power plants are very largeheat exchangers where heat from a primary fluid heated by a nuclearreactor is transferred to a secondary fluid which is converted intosteam and used to drive a turbine generator. Steam generators are housedinside a tall generally cylindrical steel shell. A large numbered ofU-shaped heat exchanger tubes are enclosed in the shell and have theirends inserted in holes formed in a horizontal tube sheet or plate nearthe bottom of the steel shell. The tubes are used to convey the primaryfluid which has been heated in the nuclear reactor. The secondary fluidor feed water used to generate the steam is introduced into the steamgenerator in such a manner that the secondary fluid flows around theoutside of the heated tubes thereby converting much of the secondaryfluid into steam which is allowed to exit the steam generator through anoutlet nozzle at the top of the steel shell.

In the past, steam generator tubing in nuclear plants have been exposedto extreme operating conditions and were susceptible to stress corrosioncracking, mechanical wear, thinning and pitting. To address thissusceptibility, a number of techniques have been developed to inspectsteam generator tubing for degradation prior to tubing failure in orderto prevent forced outages. Steam generator tubing has been most commonlyinspected using a variety of eddy current methods, most involving probeswhich were inserted into the tubes from the underside of the tube sheeton the primary side of the steam generator. The probes were insertedthrough a steam generated manway in the lower hemispherical inlet andoutlet sides of the generator below the tube sheet and into the tubesheet whereby the corresponding tubes were mapped by inserting theprobes up through the tubes.

Though highly accurate, the eddy current method of inspecting steamgenerator tubing is relatively slow and expensive, in that it is timeconsuming, requires drainage of the primary side of the generator andincreases the exposure of inspection personnel to radiation by openingup the primary side of the steam generator.

While there have been a number of attempts to use ultrasonic techniquesfor inspecting steam generator tubing, as explained in U.S. Pat. No.5,767,410, in general, most of these techniques used the Lamb ultrasonicwave method of inspection to supplement the eddy current method. A mainadvantage of the Lamb wave method is that it is not a “spot” techniquefor tubing inspection as are the eddy current methods. Using Lamb waves,a defect can be detected at relatively long distances from the probe.The range of an ultrasonic Lamb wave probe depends on the wave mode, theinformation about the defect sought, and the probe design used. Theultrasonic Lamb wave method is attractive because the attenuation ofLamb waves in a metal medium is exceptionally low. The Lamb waves canpropagate for a relatively long distance without losing much energy.Lamb waves of a typical amplitude can still be readily detected aftertraveling a distance of about 10 meters. Another important feature isthat Lamb wave propagation is not sensitive to relatively smooth changesin the tubing diameter or the tube bend, such as expansion transition,dents and U-bends.

With the replacement of the majority of the older nuclear steamgenerators with new designs utilizing thermally treated I690 manyutilities are opting for longer intervals between tubing inspections,thus not having to open the primary side of the steam generator duringoutages. This has significant cost savings. The maintenance that isperformed on the steam generator during intervals between primary sideinspections takes place from the secondary side of the unit. Typically,this involves looking for loose parts and/or cleaning of deposits fromthe secondary side. If a loose part is found, it is removed. However, onoccasions where the part is in intimate contact with the tube, wear mayhave occurred on the tube. Often, the visual inspection techniquesavailable to characterize the wear are incapable of determining thedifference between superficial removal of the deposits on the tube andsignificant wear. The narrow tube lanes, i.e., within the order of 3 mmclearance, makes it extremely difficult to characterize the depth andseverity of a wear scar to an extent that would provide sufficientconfidence that a wear mark would not develop into a leak. Thus, when awear scar is identified on the secondary side the primary side of thesteam generator must be opened and eddy current or ultrasonic techniquesapplied from the inside surface of the tube to characterize the depth ofthe wear scars. This clearly involves significant expense. What isneeded is a technique for determining the significance of the wear scarsfrom the secondary side eliminating the need to open the primary side ofthe steam generator.

SUMMARY OF THE INVENTION

The foregoing need is satisfied by the method of this invention fornon-destructively examining the walls of heat exchanger tubes from thesecondary side of a steam generator while the primary side is watersolid. The method includes the steps of contacting the outer surface ofa wall of the tube with an ultrasonic transducer that transmits ahelical like Lamb wave into the wall and analyzing reflected or alteredLamb waves for defects within the tube wall. The ultrasonic transducercan be positioned on the tube being examined at the tube sheet, on theU-bend or on an intermediate location and the helical like Lamb wave canbe transmitted unidirectionally or bidirectionally in a pseudo helicalpattern along the length of the tube. The focus of the helical Lamb wavetravels a first axial distance while making a 360° rotation around thewall of the tube and preferably the ultrasonic transducer is movedaxially along the outer surface of the tube a distance equal to at leastthe first axial distance while transmitting intermittent Lamb waves. Theultrasonic transducer or a second ultrasonic transducer receivesreflected or altered ultrasonic waves in an interval betweentransmissions of the outgoing helical Lamb waves. In another preferredembodiment the ultrasonic transducer is moved axially in increments andthe helical Lamb wave is transmitted substantially at each increment.

In still another preferred embodiment the method includes the step oftuning the frequency of the Lamb wave to sweep across a given band offrequencies to focus on different points in the tube wall. The givenband of frequencies preferably extends substantially on either side of aprimary frequency that may range between 700 KHz and 2 MHz. Thefrequency band extends approximately five percent and more preferablyone percent on either side of the primary frequency.

In one preferred embodiment the outside of the steam generator heatexchanger tubes are visually examined with a small remote camera thatcan be manipulated between the heat exchanger tubes in the tube bundle.The location and shape of wear scars are recorded and mapped. Alaboratory mockup of a steam generator heat exchanger tube filled withwater is constructed with a wear scar that corresponds in location andform to that recorded during the visual inspection. A transducer iscoupled to the tube mockup at a location remote from the wear scar andmoved relative to the tube mockup while the transducer's frequency andthe angle of introduction of the ultrasonic signal is varied until amode is identified that results in a response that is least affected bythe water in the tube and best characterizes the wear scar. Thetransducer is then applied to the outside of the steam generator tube onwhich the wear scar was originally, visually detected, preferably biasedagainst the tube and operated in the mode identified in the laboratoryto characterize the wear scar. The Lamb wave mode for the various modescar signatures or shapes are then recorded in a library that can besearched when additional wear scars are encountered so that theidentified modes can be used for subsequently detected wear scars of thesame class.

In still another embodiment the ultrasonic transducer is either a phasedarray transducer or an EMAT transducer. Preferably the transducer has abiasing mechanism that can leverage off of adjacent tubes to pressurethe transducer against the tube being interrogated to maximize thecoupling of the ultrasonic signal through the tube/transducer interface.The biasing mechanism may, for example, be a spring pack or aninflatable bladder.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a cross sectional view of a U-tube steam generator for whichthis invention can be applied;

FIG. 2 is a schematic representation of a steam generator tube bundleshowing the delivery arms and the camera and transducer assemblies beingapplied to a steam generator heat exchanger tube;

FIG. 3 is a planned view of a section of a steam generator tube showingthe transducer contacting a portion of the tube circumference on oneside and being bonded to a delivery arm on the other side;

FIG. 4 is a view of a transducer mounted on a delivery arm showing thesignal cable and couplant line that supplies a coupling medium betweenthe transducer and the tube at their interface; and

FIG. 5 is a block representation of the transducer system of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, steam generators used in nuclear reactor power plantsare very large heat exchangers in which heat from a primary fluid heatedby the nuclear reactor is transferred to a secondary fluid “water” whichis converted into steam and used to drive a turbine generator. FIG. 1illustrates such a steam generator. The steam generator 1 comprises asteel shell 2 of generally cylindrical shape having a large upper steamsection 3, a middle section 4, and a lower channel head section 5. Ahorizontal circular tube sheet or plate 6 is attached to the steel shell2 and separates the lower channel head section 5 from the middle section4. A vertical dividing plate 7 in the channel head section 5 is attachedto the tube sheet 6 and at its bottom to the channel head 5 and servesto divide the channel head section 5 into a primary fluid inlet plenum 8and a primary fluid outlet plenum (not shown). A pair of man holes 9provide access to the channel head section 5, as required.

The cylindrical middle section 4 of the steam generator 1 contains largenumbers of U-shaped heat exchanger tubes 11 which are assembled into atube bundle 12 and attached at their ends to openings in the tube sheet6. A plurality of vertically spaced, horizontal support plates orbaffles 13 have openings therein similar to those in the tube sheet 6 tohold the tubes in a proper vertical alignment. Large openings are alsoprovided in the support plates or baffles 13 to allow the secondaryfluid and steam to flow upward through the tube bundle 12 of the steamgenerator 1.

Secondary fluid or feedwater is introduced into a feedwater inlet nozzle14 located in the lower portion of the middle section 4 of the steelshell 2 above the tube sheet 6. Feedwater inlet nozzle 14 is connectedto a feedwater riser pipe 15 positioned between the inside surface ofthe steel shell 2 and the outside of the cylindrical tube bundle wrapper16 that is spaced inwardly from the inside surface of the steel shell 2by spacers 10. Feedwater riser pipe 15 extends up the length of themiddle section 4 and into the enlarged upper steam section 3 where it isconnected to a circular feedwater distribution ring 17 provided with aplurality of spray nozzles 20, which spray the secondary fluid orfeedwater into a recirculating pool 28 which receives the drain waterfrom the steam separators 25,26. The steam separators 25,26 respectivelyform the primary and secondary dryers for separating moisture from thesteam before the steam is conveyed out the steam flow outlet nozzle 27to the turbine generators. By introducing the feedwater in therecirculating pool 28, the cooled incoming feedwater is allowed to mixwith the hot recirculating water and the resulting rise in feedwatertemperature greatly reduces the thermal shock on the system and itscomponents.

Although not shown in FIG. 1 the steel shell 2 also includes manholecovers in the lower part of the middle section 4 through whichmaintenance access can be obtained. Maintenance on the secondary sidemainly involves removing sludge from the tube sheet that had beendeposited over time as a result of the change of phase from water tosteam. The heat exchanger tubes in the first generation of nuclearU-tube steam generator were highly susceptible to stress corrosioncracking. The integrity of these tubes had to be inspected atpractically every outage and were nondestructively tested by insertingeddy currents detectors through the manholes 9 and into the inletchannel 8 and through the tube sheet 6 to the individual tubes 11. Theeddy current detectors were then mapped around and along the tubes tolocate flaws. If individual tubes were determined to be defective theywere then either sleeved or plugged to avoid the possibility of primarycoolant leaking into the secondary system, to maintain the primarycoolant barrier to minimize exposure to radiation. The eddy currentinspection process was extremely time consuming and expensive. Itnecessitated the drainage of the primary coolant loop so the inlet andoutlet channels of the steam generator could be accessed and exposed themaintenance personnel to increased amounts of radiation.

With the replacement of the majority of the older nuclear steamgenerators with the new designs utilizing thermally treated I690 manyutilities are opting for longer intervals between tubing inspections.The longer time period between inspections meant that it was notnecessary to open the primary side of the steam generator during everyoutage. This has significant cost savings. The maintenance that isperformed on the steam generators during intervals between eddy currentinspections takes place from the secondary side of unit. Typically, thisinvolves looking for loose parts and/or the cleaning of deposits fromthe secondary side. As mentioned above, if a loose part is found, it isremoved. However, on occasions where the part is in intimate contactwith the tube, wear may have occurred on the tube. Often, the visualinspection techniques available to characterize the wear are able todetect the wear scars and identify their location and shape but areincapable of determining the difference between a superficial removal ofthe deposits on the tube and significant wear. The consequence is thatthe primary side of the steam generator must be opened and eddy currentor ultrasonic techniques applied from the inside surface of the tube tocharacterized the depth of the wear scars. This clearly involvessignificant expense. What is needed is a technique for determining thesignificance of the wear scars from the secondary side, eliminating theneed to open the primary side of the steam generator. However,interrogation of the wear scars to determine their significance iscomplicated by the narrow clearance in the lanes in between the tubes inthe tube bundle 12. A typical tube lane clearance is in the order ofslightly more than 3 mm. This invention satisfies that need by providingan ultrasonic technique that can be applied to the outside of the steamgenerator heat exchanger tubes while the tubes are filled with water.

When ultrasonic waves are introduced into structures where a dimensionof the structure is comparable to the wave length of the sound, modes ofpropagation different from those in the bulk material can occur. In thecase of tubing, the dimension that can be comparable to the wave lengthis the wall thickness. Some of the modes have minimal interaction withmaterial in contact with the tube surface while other modes interactstrongly. For this application, the inspection element will be placed onthe outside of the tube while the inside of the tube is filled withwater. A mode, therefore, that has minimal interaction with the presenceof the water is the most desirable. Further, the modes of propagationwill depend upon the wall thickness and the frequency of the ultrasonicwave. The speed of propagation will be a function of the mode and thefrequency.

FIG. 2 provides a schematic representation of a steam generator tubebundle 12 showing a piezoelectric transducer 18 mounted on a couplingwedge 19 and brought into contact with the outside of a heat exchangertube 22 by a delivery arm 21. The piezoelectric transducer 18 isemployed to excite the desired mode. The shape of the wedge and thefrequency of the ultrasonic wave generated by the transducer 18determine the mode of propagation. One limitation of this technique isthat the wall thickness of the tubing is not constant. Tubingmanufacturers allow a variation in wall thickness. Since the desiredmode of propagation depends on the frequency, wall thickness, and thewedge dimensions, an arbitrary wedge may or may not be optimal for aspecific tube. To compensate for this the transducer 18 of thisinvention is a phase array transducer. The apparent angle of incidenceis varied by appropriate pulse forming to assure that the energy fromthe transducer couples into the desired mode. With the proper mode andcoupling wedge it is possible that the wave can propagate overconsiderable distances such that a transducer assembly placed near thetube sheet could characterize wear to above the top support plate 13(shown in FIG. 1). Thus, an observed wear location need not be directlyaccessible to the ultrasonic transducer assembly. Further, while theultrasonic energy is injected into the tube at one azimuthal locationand propagates along the axis of the tube the energy spreads as itpropagates forming a spiral, pseudo helical or helical like pattern.Thus, a wear scar on the opposite side of the tube (180° away) is stilldetectable. Since the energy propagation is spiral like it is desirableto move the transducer assembly axially over the length of complete waverotation to ensure complete coverage over the entire circumference ofthe tube.

FIG. 3 shows a planned view of a cross section of the tube 22 that hasan ultrasonic piezoelectric transducer 18 in direct contract. Thetransducer 18 is held in place by a bonding material 23, which attachesthe transducer to a delivery arm 21. In the preferred embodiment it isdesirable that the profile of the combination of the transducer and thedelivery arm be equal to 3 mm or less so that the transducer can beinserted within the tube lanes.

FIG. 4 illustrates the transducer assembly that includes a piezoelectricultrasonic transducer 18 having a signal cable 24 that transmits anintermittent burst of ultrasonic energy and receives reflected signalsin between the ultrasonic transmissions. A couplant line 29 is alsoprovided for injecting a couplant such as water between the transducer18 and the tube 22 to assist the transmission of the ultrasonic energyinto the surface of the tube.

FIG. 5 schematically illustrates an inspection system having atransducer 18 coupled to a pipe 22. The transducer 18 receives a pulseoutput 31 from an InterSpec Dos with a programmable tone burst generator32. A reflected signal 30 is received between bursts and is analyzed inthe InterSpec Dos 32 to characterize the reflected signal.

The methodology of this invention relies on the interaction of thepropagating mode with the region of the tube where the wear scar hasoccurred. A significant wear scar represents a significant change inwall thickness and therefore a change in mode will occur when the wavepropagates through a wear scar region having a significant change inwall thickness. Energy from the interrogating wave will be “reflected”by the presence of the wear scar back to the transducer assembly to bedetected. The absence of the reflected response indicates that the wearis not significant and therefore does not require repair. For wear thatoccurs in the vicinity of the tube sheet it is possible to use twoultrasonic sensors, located above and below the wear operating in apitch-catch mode. In this scenario, energy is transmitted from onetransducer and received by the second transducer located on the oppositeside of the wear scar. The presence of the wear scar is detected by achange in the arrival time and presence of additional or missing modesof propagation resulting from a change in wall thickness associated withthe wear scar as the energy passes through. For both the reflected andtransmitted modes of detection, analysis of the frequency content andarrival time of the response provides information as to the potentialthrough wall extent of the wear scar.

The invention thus contemplates one or two transducer assembliescomprising the piezoelectric phase array transducers, the appropriatelydesigned wedge and the electronics to excite the transducer and todetect and interpret the received energy. The received response is thenprocessed using commercially available algorithms to separate thearrival of the energy associated with the various modes (frequency) andthe arrival time. Such commercially available algorithms, e.g., wavelettransforms, can be found in products such as LABVIEW from NationalInstrument Corp. (ni.com) and UTTEST from FBS Inc. From derived data thedepth of the wear scar can be estimated.

The invention also includes the means to bring the transducer assemblyinto contact with the appropriate tube within the steam generator andthen to move the transducer assembly axially along the tube to assurecomplete interrogation of the entire circumference of the tube. Themeans for bringing the transducer assembly into contact with theappropriate tube can be the delivery arm 21 shown in FIG. 2 or it can beany other remote positioning mechanism such as a robotic vehicle or arm.Contact with the heat exchanger tube to be interrogated can be enhancedby leveraging off of adjacent tubes using an inflatable bladder orspring pack 34 as is shown in FIG. 5. While the transducer assemblydescribed here is a piezoelectric phase array mounted on a wedge, it canalso take the form of an EMAT transducer (Electro Magnetic Air CoupledTransducer), either in a single mode or as a phase array. Preferably, tointerrogate the tube at various locations along its elongated length itis desirable that the transducer be scanned through a range offrequencies centered around a primary frequency which preferably isbetween 700 KHz and 2 MHz. The range should typically span five percenton either side of the primary frequency and more preferably one percenton either side of the primary frequency. Additionally, it should beappreciated that the ultrasonic transducer can be positioned in theU-bend region and the ultrasonic signal transmitted in oppositesdirections along the length of the tube to inspect the entire tube fromone location.

In one preferred embodiment the outside of the steam generator heatexchanger tubes are visually examined with a small remote camera, suchas the camera 36 shown in FIG. 2. The camera can be manipulated betweenthe heat exchanger tubes in the tube bundle 12 for a thoroughinvestigation of the outside surface of each of the heat exchangertubes. The location and shape of the wear scars are recorded and mapped.A laboratory mockup of a steam generator heat exchanger tube filled withwater is then constructed with a wear scar that desirably corresponds inlocation and form to that recorded during the visual inspection. Anultrasonic transducer is then coupled to the tube mockup at a locationremote from the wear scar and moved relative to the tube mockup whilethe transducer's frequency and the angle of introduction of theultrasonic signal is varied until a mode is identified that results in aresponse that is least affected by the water in the tube and bestcharacterizes the wear scar's penetration through the tube wall. Bybeing least affected by the water in the tube means that the ultrasonicsignal is least attenuated by not being coupled into the water at thetube wall/water interface. The mode being determined is defined by theangle of introduction and the frequency of the ultrasonic signal. Instill another embodiment identification of the preferred mode isdetermined by experimentally varying the transducer's frequency and theangle of introduction of the ultrasonic signal directly on the steamgenerator heat exchanger tube exhibiting the wear scar. Once the propermode is selected by the foregoing experimental technique the ultrasonictransducer can be applied directly to the steam generator heat exchangertubes that the visual inspection identified as having wear scars. TheLamb wave mode for the various wear scar signatures or shapes are thenrecorded in a library that can be searched when additional wear scarsare encountered so that the identified modes can be used forsubsequently detected wear scars of the same class, i.e. wear scars ofthe same or similar shape on the same size tube.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details could be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention, which is to be given thefull breadth of the appended claims and any and all equivalents thereof.

1. An ultrasonic transducer having a profile that configured to navigatea clearance in lanes between heat exchange tubes in a secondary side ofa nuclear steam generator wherein the clearance is approximately 3 mm,including a biasing mechanism that configured to leverage off ofadjacent heat exchange tubes to positively press the ultrasonictransducer into firm contact with a heat exchange tube to beinterrogated for wear scars.
 2. The ultrasonic transducer of claim 1wherein the biasing mechanism is a spring pack.
 3. The ultrasonictransducer of claim 1 wherein the biasing mechanism is an inflatablebladder.